(1) Field of the Invention
The present invention relates to a composite PTC material favorably used in, for example, a current-limiting element which controls fault current. ("PTC" is an abbreviation of "positive temperature coefficient of resistance".)
(2) Description of Related Art
PTC materials have a property of increasing the electrical resistance sharply with an increase in temperature in a particular temperature range. Therefore, they are used, for example, as a current-limiting element which controls fault current in a breaker.
The best known PTC material is a barium titanate type ceramic whose electrical properties change at the Curie point. With this PTC material, however, the power loss is large because of its high room temperature resistivity and, moreover, the production cost is high. Hence, other substances having PTC property were looked for.
As a result, it was found that composite materials made of a polymer (a matrix) and a conductive substance (a filler) have the same PTC property as possessed by the barium titanate type ceramic.
For example, a mixture consisting of particular proportions of a crystalline polymer (e.g. a polyethylene) as an insulator and conductive particles (e.g. carbon particles) has conductive paths formed in the polymer matrix, is very low in electrical resistance, and acts as a conductor as a result of insulator-conductor transition.
In such a composite material consisting of particular proportions of a crystalline polymer and conductive particles, since the polymer has a thermal expansion coefficient far larger than that of the conductive particles, the crystalline polymer gives rise to sharp expansion when the composite material is heated and the crystalline polymer is melted.
As a result, the conductive particles forming conductive paths in the polymer are separated from each other, the conductive paths are cut, and the electrical resistance of the composite material increases sharply and the composite material shows PTC property.
When an organic substance such as the above polymer or the like is used as a matrix in a composite PTC material, however, there has been a problem in that when high temperatures caused by fault current continue for a long time, the composite material is unable to exhibit its intended action because the organic substance is generally low in heat resistance.
Study was also made on composite materials made of a silica type substance (a matrix) such as quartz, cristobalite or the like and conductive particles. Similarly to the barium titanate type ceramic, these materials are high in room temperature resistivity and give a large power loss.
Conventional composite materials also had a problem in that they allow no repeated operation because the resistance after operation does not return to the initial resistance even if a temperature falls once the resistance rises.